Pressure Treated Lumber: What It Is and When to Use It

Pressure-treated lumber has special preservatives that make it last longer. While these preservatives can vary, most enable the wood to better resist water damage, mold, and insect infestations.

Pressure Treated Lumber

But this doesn’t mean pressure-treated lumber is always the best choice. Instead, you’ll want to use it for projects like decking, garden beds, or anywhere wood may come into contact with moisture.

If you’re considering pressure-treated lumber for your project, here’s when and why to use it.

What Is Pressure Treated Lumber?

What Is Pressure Treated Lumber

Pressure-treated lumber is the result of adding chemical preservatives to wood using a high-pressure treatment. The injection of these preservatives protects the wood from rot, moisture damage, and insect infestations. It also adds decades to the wood’s lifespan.

There are three main types of pressure-treated lumber: lumber for ground use, underground use, and marine use.

The most common chemicals used to treat pressure-treated lumber are copper azole (CA), micronized copper azole (MCA), and copper quaternary (ACQ).

Since pressure-treated lumber has liquids forced into it, it’s often wet when it arrives on-site and may take a couple of weeks to dry. Because of this, the boards you purchase at a home improvement store may experience slight shrinkage or warping.

When to Use Pressure Treated Wood (And When Not To)

When it comes to outdoor projects, there are a lot of uses for pressure-treated lumber. It’s one of the best decking and fence materials but works for other projects, too.

Here’s when to use pressure-treated lumber:

  • Decks
  • Fences
  • Garden beds
  • Docks
  • Wooden sidewalks
  • Framing of outdoor structures
  • Anywhere outdoors, wood will come into contact with moisture

Since pressure-treated wood has chemicals in it, you don’t want your food to touch it. So, if you’re using it on garden beds, line the beds with plastic first. Alternatively, you can use an oil-based sealant on the wood for a bit of added protection.

And remember, the treated wood you pick up from a home improvement store is probably still wet. So if you plan on painting or staining it, allow it to fully dry beforehand.

When to Use Pressure Treated Wood

Is It Okay to Use Pressure Treated Lumber Inside?

Here’s the bad news: pressure-treated wood can possibly leach chemicals into its environment. So even though today’s lumber is less toxic than in years past, its chemical treatment still poses a small risk when used indoors.

There’s a bit of debate as to whether you should use it inside. 

Some builders advocate using pressure-treated lumber as bathroom subfloors or around interior surfaces with high condensation levels.

If you do use it inside, the most important thing is to avoid using it on food-safe surfaces like a cutting board or countertops. Also, avoid using it anywhere pets may chew on it.

Types of Pressure Treated Lumber

Types of Pressure Treated Lumber

When you visit a home improvement store like Lowes or Home Depot, you’ll find different types of pressure-treated wood: above-ground and ground contact.

Above ground pressure treated wood is for applications in which the lumber will be at least six inches off the ground. You should only use it where maintaining and replacing it will be easy. It needs to be in a well-ventilated area.

Ground contact pressure-treated lumber is twice as durable as above-ground pressure-treated wood. You can use it above ground or in direct contact with soil or concrete. (Think fence posts, deck posts, and garden beds.)

Use ground contact pressure-treated wood anywhere your boards won’t be easy to replace or maintain.

Grades of Pressure Treated Lumber

Grades of Pressure Treated Lumber

Aside from the two main types of pressure-treated wood, there are also two grades: grade 1 and grade 2.

These grades are assigned based on the appearance of the wood. Grade 1 is the better-looking wood and is what you should use if your boards will be visible. Wood with a grade 2 label will have more cosmetic blemishes and is a good choice for boards that won’t be visible.

Pressure Treated Lumber Sizes

If you’re ready to tackle an outdoor project, you’re probably wondering what sizes you can find.

Here are the most common pressure-treated lumber sizes available at home improvement stores:

Listed Size (Inches)Approximate Actual Dimensions (Inches)Length (Feet)
1 x 2¾ x 1 ½ 8
1 x 4¾ x 3 ½ 8,12
1 x 6¾ x 5 ½ 6, 8, 10, 12
1 x 8¾ x 7 ¼ 8
2 x 21 ½ x 1 ½ 8
2 x 41 ½ x 3 ½ 4, 8, 10, 12, 16
2 x 61 ½ x 5 ½ 8, 10, 12, 16
2 x 81 ½ x 7 ¼ 8, 10, 12, 16, 20
2 x 101 ½ x 9 ¼ 8, 10, 12, 16, 20
2 x 121 ½ x 11 ¼ 8, 10, 12, 16, 20
4 x 43 ½ x 3 ½ 8, 10, 12, 16
4 x 63 ½ x 3 ½ 8, 10, 12
6 x 65 ½ x 5 ½ 8, 10, 12

Frequently Asked Questions (FAQ)FAQ

How long will pressure-treated wood last?

The lifespan of pressure-treated lumber will depend on the climate and how well you maintain it. For example, posts in the ground may last as long as 40 years, especially when covered by a structure like a deck. But fencing or deck boards may last only ten years, depending on the climate.

What are the disadvantages of using pressure-treated wood?

While there are many pros to using pressure-treated wood, there are also a few cons. Pressure-treated lumber contains chemicals that can possibly leach into the environment. It’s not recyclable, unsuitable for indoor use, and may warp when dry because of the high-pressure treatment process.

Does pressure-treated lumber rot?

Pressure-treated lumber can rot over long periods or if you use the wrong type of wood. For example, if you use above-ground pressure-treated wood in the ground, it will rot in only a year or two. You need ground contact pressure-treated wood for anything that will be 6 inches or closer to the ground.

Do I need to seal pressure-treated wood?

You should seal pressure-treated wood on most projects, especially a deck or fence. Adding a sealant or painting pressure-treated wood provides an extra layer of protection against moisture and insect infestations, making your wood last longer. You can seal garden bed lumber with an oil-based sealant for protection against chemical leaching.

Can I put pressure-treated wood in the ground?

Yes, you can put pressure-treated wood in the ground. Just make sure to purchase ground-contact wood. Ground contact pressure-treated lumber has twice the preservatives to make it more durable.

Final Thoughts

Pressure-treated lumber undergoes a process using high pressure to inject it with chemicals that make it more durable. If you’re working on any outdoor project, use pressure-treated lumber so that your wood doesn’t rot, mold, or become infested with insects.

There are two types of this lumber: above-ground use and ground contact use. There are also grades 1 and 2, which symbolize the wood’s cosmetic appearance. As far as pressure-treated lumber prices, those will vary depending on the store and current supply and demand.